IBD is real hot on having sell rules (knowing when which to sell) as well as buy rules. One sell rule that I like is selling after a 20% jump to 'lock in the gains; they fudge it to say if it soars say, 50-100% to wait a few weeks to stabilize before selling. One big sell rule is to dump if it reaches new highs is lighter trading --meaning demand is fading. Another good rule (I use a lot) is to dump when it falls some percent (mine is 10%) below its all time high.
Their chart seems to show them hitting all the sell rules. Something else, fwiw IBD now rates GTAT at only 56 (out of 100); even though institutions had been buying it up it's been showing lately poor sales and earnings growth.
Bottom line is that your picking GTAT last year and looking hard at it now is brilliant. Please tell me what else you're considering buying now...